1. Field of the Invention
The present invention relates to digital converter apparatus for improving the output of a two-wire transmitter sensing a process variable.
2. Summary of the Invention
An existing analog two-wire transmitter comprises a sensor module means coupled to a process variable for sensing and for providing a sensor output as a function of the process variable. The existing transmitter further comprises excitation means coupled to the sensor module means for providing excitation thereto. The existing transmitter further comprises analog detector means for providing analog conversion of the sensor signal to a two-wire transmitter output representative of the sensed process variable. The existing transmitter is modified such that the transmitter's output is improved. The analog detector means are removed from the transmitter and replacement apparatus which digitally calculates the transmitter's output are disposed in the transmitter. The replacement apparatus receive the sensor output and provide linearity or other correction to the output. In a preferred embodiment, the existing excitation means are removed and the apparatus comprise replacement excitation means which are disposed in the transmitter and coupled to the sensor module means for providing excitation thereto. In a further preferred embodiment the sensor module means comprises at least one capacitive sensor for sensing the process variable, rectification means coupled to the sensor output for providing rectification thereto, and analog correction means for providing analog corrections to the sensor.
In yet a further preferred embodiment, the apparatus comprises a microprocessor calculating output correction, span, and zero adjustments.
The existing transmitter can thus have its output improved while the transmitter remains in situ and coupled to the process variable and the loop. A transmitter with a digitally corrected output is thus provided without replacement of the existing sensor module or decoupling of the transmitter from process lines or the two-wire loop.